Non-uniform minority carrier lifetime distributions in high performance silicon power devices

ABSTRACT

This invention is directed to a process for heat-treating a single crystal silicon segment to influence the profile of minority carrier recombination centers in the segment. The segment is subjected to a heat-treatment to form crystal lattice vacancies, the vacancies being formed in the bulk of the silicon. The segment is then cooled at a rate which allows some, but not all, of the crystal lattice vacancies to diffuse to the front surface to produce a segment having the desired vacancy concentration profile. Platinum atoms are then in-diffused into the silicon matrix such that the resulting platinum concentration profile is substantially related to the concentration profile of the crystal lattice vacancies.

REFERENCE TO RELATED APPLICATION

This is a divisional of application Ser. No. 10/911,965, filed on Aug.5, 2004; which is a divisional of application Ser. No. 09/366,850, filedAug. 4, 1999, now U.S. Pat. No. 6,828,690; which claims the benefit ofU.S. provisional application Ser. No. 60/095,407, filed on Aug. 5, 1998and U.S. provisional application Ser. No. 60/098,808, filed on Sep. 2,1998; all of which are incorporated in their entirety herein byreference.

BACKGROUND OF THE INVENTION

The present invention generally relates to a process for the preparationof high performance silicon power devices having improved turn-off orswitching time, as well as forward voltage drop. More specifically, theprocess of the present invention relates to the preparation of a siliconsegment containing regions having different minority carrier lifetimesby means of controlling the concentration profile of recombinationcenters within the silicon segment.

For designers of certain types of solid state power devices, such asthyristors or power diodes, both the switching speed and turn-off chargeare important considerations. As the switching speed increases and theturn-off charge decreases, the more efficient the device becomes.Unfortunately, however, conventional methods of increasing the switchingspeed of a particular device often result in an appreciable increase inturn-off charge, or forward voltage drop, which acts to hinder deviceefficiency.

Typically, in the “on” state, power devices are flooded with excesscarriers which are responsible for carrying the large current that isrequired. Problems arise with such devices, however, when the devicesare switched off; more specifically, problems arise with how to get ridof these carriers when they are no longer needed. Minority carrierrecombination has been identified as one of the major mechanisms bywhich these excess carriers may be dissipated. The faster recombinationoccurs, the faster a power device can be depleted of carriers whenswitched off and, therefore, the faster the device actually switchesoff. However, if the recombination rate is high throughout the bulk ofthe device, then a higher voltage is required to support the currentwhen the device is on. As a result, the power consumption of the deviceis increased and, accordingly, the efficiency of the device isdecreased. Therefore, any improvement in the switching speed of thedevice is achieved at the cost of decreased device efficiency.

It is known in the art that doping semiconductor devices with lifetimekilling impurities (i.e., recombination centers such as gold orplatinum) result in an increase in the recombination rate when thedevice is turned off, and thus an increase in the switching speed aswell. (See, e.g., V. Temple and F. Holroyd, “Optimizing Carrier LifetimeProfile for Improved Trade-off Between Turn-off Time and Forward prop,”IEEE Transactions on Electron Devices, ed. 23, pp. 783-790 (1983).) Inthe past, such impurity doping has typically been applied to large areasof the device, even throughout the entire bulk of the device. Thisapproach has resulted in significant decreases in the device turn-offtime. However, accompanied with this improvement is an increase indevice forward voltage drop. Similar results have been obtained whenalternative methods of “lifetime killing” have been employed, includingelectron, proton and gamma radiation, throughout the bulk of the device.

In an attempt to avoid the problems associated with bulk doping or bulktreatment of the device, local lifetime killing has been proposed. (See,e.g., Temple et al., IEEE Transactions on Electron Devices, pp.783-790.) For example, local regions of a thyristor have beenselectively irradiated, or doped with gold, in an attempt to control thelocation of the minority carrier recombination centers, and thusdecrease the minority carrier recombination lifetime within a specificregion of the device. Such approaches are attractive because, in theoryat least, they allow for a region to be selectively doped withrecombination centers, thus improving the switching speed within thisregion, while leaving the bulk of the device undoped, and thus preventthe large forward voltage drop associated with bulk doping or treatmentof the device.

Previously, optimizing the spatial location of these recombinationcenters within the bulk of the device has been considered. For example,as illustrated in FIG. 1A and FIG. 1B, Temple et al. demonstrated thedesirability of having a region of enhanced recombination (i.e., shortminority carrier lifetime) within the device in a plane which isperpendicular to the direction of the on-state current flow. However,the practical problem of how to selectively tailor or control thelocation of dopants within the device has to date proven difficult. Infact, Temple et al. state that the local tailoring of such a regionwithin a device would not be easily achievable experimentally, and thatany practical applications would involve an extensive developmentprogram with an unknown chance of success.

Accordingly, a need continues to exist for a process whereby theconcentration of minority carrier recombination centers within a devicemay be selectively profiled or tailored such that these centers may beprimarily located within a specific region, with the remainder of thedevice being substantially free of such centers.

SUMMARY OF THE INVENTION

Among the objects of the invention, therefore, is the provision of asingle crystal silicon segment suitable for use in the fabrication of asolid state power device having increased switching speed without theattendant increase in forward voltage drop; the provision of such asilicon segment which contains a non-uniform depth distribution ofminority carrier recombination centers; the provision of such a segmentwhich contains a region having an improved minority carrierrecombination rate; the provision of a process for manufacturing such asilicon segment in which the segment undergoes an initial thermaltreatment in order to profile the vacancy concentration therein; and,the provision of such a process in which interstitial platinum atoms arein-diffused into the silicon segment under conditions which result in aplatinum concentration profile generally corresponding to the vacancyconcentration profile.

Briefly, therefore, the present invention is directed to a singlecrystal silicon segment having two major, generally parallel surfaces,one of which is the front surface of the segment and the other of whichis the back surface of the segment, a central plane between the frontand back surfaces, a circumferential edge joining the front and backsurfaces, a surface layer which comprises the region of the segmentbelow the front surface and a distance, D₁, as measured from the frontsurface and toward the central plane, and a bulk layer which comprises asecond region of the segment between the central plane and the firstregion. The segment is characterized in that it has a non-uniformdistribution of minority carrier recombination centers, with theconcentration of the centers in the bulk layer being greater than theconcentration in the surface layer and with the centers having aconcentration profile in which the peak density of the centers is at ornear the central plane with the concentration generally decreasing fromthe position of peak density in the direction of the front surface ofthe segment.

The present invention is further directed to a single crystal siliconsegment having two major, generally parallel surfaces, one of which isthe front surface of the segment and the other of which is the backsurface of the segment, and a central plane between the front and backsurfaces. The segment is characterized in that it has a non-uniformdistribution of minority carrier recombination centers between the frontand back surfaces, wherein a maximum concentration of the recombinationcenters is in a region which is between the front surface and thecentral plane and nearer to the front surface than the central plane,the concentration of the recombination centers increases from the frontsurface to the region of maximum concentration and decreases from theregion of maximum concentration to the central plane.

The present invention is further directed to a process for heat-treatinga single crystal silicon segment to influence the concentration profileof minority carrier recombination centers in the segment. The siliconsegment has a front surface, a back surface, a central plane between thefront and back surfaces, a surface layer which comprises the region ofthe segment between the front surface and a distance, D, measured fromthe front surface and toward the central plane, and a bulk layer whichcomprises the region of the segment between the central plane andsurface layer. The process comprises heat-treating the segment in anatmosphere to form crystal lattice vacancies in the surface and bulklayers, controlling the cooling rate of the heat-treated segment toproduce a segment having a vacancy concentration profile in which thepeak density is at or near the central plane with the concentrationgenerally decreasing in the direction of the front surface of thesegment, and thermally diffusing platinum atoms into the silicon matrixof the cooled segment such that a platinum concentration profile resultswhich is substantially dependant upon the vacancy concentration profile.

The present invention is still further directed to a process forheat-treating a single crystal silicon segment to influence theconcentration profile of minority carrier recombination centers in thesegment. The silicon segment has a front surface and a back surface, thefront surface having only a native oxide layer present thereon, and acentral plane between the front and back surfaces. The process comprisesheat-treating the front surface of the segment in a nitriding atmosphereto form crystal lattice vacancies in the segment, and then controllingthe cooling rate of the heat-treated segment to produce a vacancyconcentration profile in the cooled segment in which a maximumconcentration is between the front surface and the central plane andnearer to the front surface than the central plane, the vacancyconcentration generally increasing from the front surface to the regionof maximum concentration and generally decreasing from the region ofmaximum concentration to the central plane. Platinum atoms are thenthermally diffused into the silicon matrix of the cooled segment suchthat a platinum concentration profile results which is substantiallydependant upon the vacancy concentration profile.

Other objects and features of this invention will be in part apparentand in part pointed out hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic diagram of a thyristor structure containing asingle low lifetime region; that is, a region having a higherconcentration of minority carrier recombination centers relative to theother regions of the thyristor structure.

FIG. 1B is a plot of a preferred concentration profile of minoritycarrier recombination centers (i.e., a minority carrier lifetimeprofile) in the thyristor structure of FIG. 1A, during turn off.

FIG. 2 is a plot generally depicting the minority carrier recombinationcenter concentration relative to the depth of a silicon segment(extending from one surface of the segment to the other, but excludingthe surfaces themselves), in accordance with the present process.

FIG. 3 is a plot of the platinum concentration versus the depth of thesilicon segment (extending from one surface to the other, but excludingthe surfaces themselves), showing in detail the platinum concentrationprofile as a function of treatment time when the Frank-Turnbulldiffusion mechanism is dominant and the vacancy concentration profile isgenerally uniform.

FIG. 4 is a plot of the platinum concentration versus the depth of thesilicon segment (extending from one surface to the other, but excludingthe surfaces themselves), showing in detail the platinum concentrationprofile as a function of treatment time when the “kick-out” mechanism isdominant and the vacancy concentration profile is generally uniform.

FIG. 5 is a schematic depiction of the process of the present invention.

FIGS. 6A through 6D are plots generally depicting the platinumconcentration relative to the depth of the silicon segment (extendingfrom one surface to the other, but excluding the surfaces themselves),showing in detail the various platinum concentration profiles which maybe achieved as a result of the different embodiments of the process ofthe present invention.

FIG. 7 is a plot of the platinum concentration versus the depth of thesilicon segment (extending from one surface to the other, but excludingthe surfaces themselves), showing in detail the difference profileswhich may be obtained if an enhanced oxide layer is present (curve B) oronly a native oxide layer is present (curve A) during the thermalannealing treatment.

FIGS. 8A through 8F are plots generally depicting the platinumconcentration relative to the depth of the silicon segment (extendingfrom one surface to the other, but excluding the surfaces themselves),showing in detail the various asymmetric platinum concentration profileswhich may be achieved as a result of the different embodiments of theprocess of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In accordance with the process of the present invention, a siliconsegment suitable for use in the preparation of electronic power devices,such as thyristors and power diodes, may be prepared to contain a peakconcentration of recombination centers within a specific region, orregions, of the segment, while the remainder of the silicon segmentcontains a concentration of recombination centers which is comparativelylower. The present invention is further characterized by the fact thatplatinum atoms which are diffused into the silicon matrix assume aconcentration profile that substantially corresponds to the vacancyconcentration profile, thus establishing a recombination centerconcentration profile throughout the silicon segment.

Without being held to any particular theory, it is generally believedthat, to a first order, the minority carrier recombination lifetime isinversely proportional to the concentration of silicon lattice defectswhich create discreet energy levels in the bandgap of a semiconductor.These levels offer an energetic “stepping stone” for recombination of anexcess carrier across the band gap, thus increasing the rate at whichthis occurs. The typical parameter which is used to characterize thisrate is the minority carrier recombination lifetime, τ. The basicminority carrier lifetime of a semiconductor τ₀ may be expressed byEquation (1):τ₀=(V _(th) σN _(T))  (1)

where

V_(th) is the thermal velocity of the minority carrier;

σ is the capture cross section of the recombination center for theminority carrier; and,

N_(T) is the density of the recombination centers.

For a given semiconductor, the only adjustable parameter in the minoritycarrier lifetime equation is the recombination center density, orconcentration. Therefore, to achieve the lifetime profile as suggestedby FIG. 1B (where N is the number of carriers present, N is the averagecarrier concentration on the on state, and N/N is the normalizedconcentration thereof), a layer of material is required somewherebetween the two surfaces of the silicon segment with a higherconcentration of recombination centers than in both of the surfaceregions. A desired concentration profile for these recombination centersis schematically illustrated in FIG. 2.

Recombination centers may be introduced into a semiconductor by a numberof means, the most common of which being the diffusion of a foreignelement, typically a metal, into the silicon material. In recent years,much has been learned about the diffusion and incorporation mechanism ofplatinum, a recombination center, in silicon power device technology.Experience to date suggests platinum is not incorporated into silicon bythe typical substitutional or interstitial diffusion mechanisms, as isthe case for boron and iron, respectively. Rather, platinum is believedto be incorporated through one of two possible mechanisms whereby a fastdiffusing, low solubility interstitial metal species is transformed intoa slow diffusing, high solubility substitutional species by interactingwith intrinsic point defects, i.e., crystal lattice vacancies andsilicon self-interstitial. For purposes of recombination, or minoritycarrier lifetime, it is the resulting concentration of thesubstitutional species, i.e., platinum, which is important to thepresent invention.

The first mechanism through which a platinum atom and an intrinsic pointdefect may interact involves the combination of an interstitial platinumatom with a vacancy to produce a substitutional platinum atom. Thismechanism is typically referred to as the “Frank-Turnbull” (i.e., “F-T”)mechanism, and may be expressed by Equation (2):V+M_(i)

M_(s)  (2)

where

V is a silicon lattice vacancy;

M_(i) is an interstitial platinum atom; and,

M_(s) is a substitutional platinum atom.

The second mechanism occurs when an interstitial platinum atom “kicks” asilicon atom out of its lattice site and into an interstitial site. Thismechanism, i.e., the “kick-out” mechanism, may be expressed by Equation(3):M_(i)

M_(s)+I  (3)

where

I is a silicon self-interstitial atom;

M_(i) is an interstitial platinum atom; and,

M_(s) is a substitutional platinum atom.

For a given attempt to in-diffuse platinum, which mechanism willdominate depends, at least in part, on the diffusion time andtemperature, and the concentration of vacancies and self-interstitialspresent in the sample. If the F-T mechanism dominates, then a platinumconcentration profile such as that depicted by FIG. 3 may be formed orcreated in the silicon segment. It may be observed from Equation (2)that if the F-T mechanism is followed, the platinum concentrationreached in the center of the sample is about equal to the vacancyconcentration at the center. This relationship may be expressed byEquation (4) as:C _(m) =C _(v)/(1+C ^(eq) _(v)/C^(eq) _(m))  (4)

where

C_(m) and C_(v) are the concentrations of metal atoms and vacancies,respectively; and,

C^(eq) _(v) and C^(eq) _(m) are the equilibrium values of the vacancyand metal concentrations at the diffusion temperatures, respectively.

Typically, C^(eq) _(v) is much greater than C^(eq) _(m) and, therefore,C_(m)≈C_(v) (i.e., C_(m) is about equal to C_(v)) As a result, when theF-T mechanism is followed, interstitial platinum atoms which arein-diffused fill the existing vacancies in the sample and becomesubstitutional in the process.

Referring again to FIG. 3, it can be seen that the interaction betweeninterstitial platinum atoms and vacancies to produce substitutionalplatinum occurs rapidly; that is, as can be seen from FIG. 3, transportof interstitial platinum atoms is sufficiently fast such that thisinteraction is essentially time independent. Furthermore, it is to benoted from FIG. 3 that while C_(m)≈C_(v) in the bulk of the siliconsegment, there is some deviation from C_(m)≈C_(v) in regions of thesilicon segment near the segment surfaces.

Referring now to FIG. 4, it can be seen that if the results of thekick-out mechanism (FIG. 4) and the F-T mechanism (FIG. 3) are compared,the F-T mechanism provides a more uniform depth distribution ofrecombination centers, from the perspective of the desired concentrationprofile shown in FIG. 1B. However, it is the fact that platinum atomsincorporated by the F-T mechanism appear to be closely related to thevacancy concentration profile which is the focus of the presentinvention. If the vacancy concentration profile of the silicon materialcan be controlled, then the concentration profile of platinum in thedevice may also be controlled, provided in-diffusion of the platinumatoms follows the F-T mechanism.

Accordingly, the process of the present invention affords the means bywhich to reproducibly control the concentration profile of recombinationcenters in the silicon material by advantageously controlling thevacancy concentration profile.

More specifically, by controlling the vacancy concentration profilewithin the silicon material and the conditions under which platinum isin-diffused into the material, such that the F-T mechanism is followed,recombination centers may be incorporated within the material at adesired depth distribution and thereby provide optimum deviceperformance.

The starting material for the process of the present invention istypically a segment of single crystal silicon which has been sliced froma single crystal ingot grown in accordance with conventional Czochralskicrystal growing methods. Alternatively, however, the segment of singlecrystal silicon may be obtained from an ingot grown in accordance withconventional Float-zone crystal growing methods. Such methods, as wellas standard silicon slicing, lapping, etching, and polishing techniquesare disclosed, for example, in F. Shimura, Semiconductor Silicon CrystalTechnology, Academic Press, 1989, and Silicon Chemical Etching, (J.Grabmaier ed.) Springer-Verlag, New York, 1982 (incorporated herein byreference).

It is to be noted that because the process of the present inventionproceeds in accordance with the F-T mechanism, interstitial platinumatoms fill vacancies in the silicon matrix upon in-diffusion. Therefore,the platinum concentration profile is a function of, or substantiallydependent on, the vacancy concentration profile within the siliconsegment. Stated another way, the position assumed by platinum atomswithin the silicon matrix correspond to the positions of vacancieswithin the matrix. Gettering by oxygen precipitants is therefore notbelieved to be involved in the present process. As a result, the oxygenconcentration of the silicon segment is not narrowly critical to theprocess of the present invention. Accordingly, the starting siliconsegment may contain essentially no oxygen, or it may have an oxygenconcentration falling anywhere within or even outside the rangeattainable by the Czochralski process (which typically ranges from about5×10¹⁷ to about 9×10¹⁷ atoms/cm³, as determined by ASTM standardF-121-83).

For Czochralski grown silicon, depending upon the cooling rate of thesingle crystal silicon ingot from the temperature of the melting pointof silicon (about 1410° C.) through the range of about 750° C. to about350° C., oxygen precipitate nucleation centers may be formed. Thepresence or absence of these nucleation centers in the starting materialis not critical to the present invention. Preferably, however, thesecenters are capable of being dissolved by heat-treating the silicon attemperatures not in excess of about 1300° C. Certain heat-treatments,such as annealing the silicon at a temperature of about 800° C. forabout four hours, can stabilize these centers such that they areincapable of being dissolved at temperatures not in excess of about1150° C.

Substitutional carbon, when present as an impurity in single crystalsilicon, has the ability to catalyze the formation of oxygen precipitatenucleation centers. For this and other reasons, therefore, it ispreferred that the single crystal silicon starting material have a lowconcentration of carbon. That is, the single crystal silicon preferablyhas a concentration of carbon which is less than about 5×10¹⁶ atoms/cm³,preferably which is less than 1×10¹⁶ atoms/cm³, and more preferably lessthan 5×10¹⁵ atoms/cm³.

Referring now to FIG. 5, the starting material for the present inventionis preferably a single crystal silicon segment 1 having a front surface3, a back surface 5, and an imaginary central plane 7 between the frontand back surfaces. The terms “front” and “back” in this context are usedto distinguish the two major, generally planar surfaces of the segment;the front surface of the segment as that term is used herein is notnecessarily the surface onto which an electronic device willsubsequently be fabricated nor is the back surface of the segment asthat term is used herein necessarily the major surface of the segmentwhich is opposite the surface onto which the electronic device isfabricated. In addition, because silicon segments may have some totalthickness variation (TTV), warp and bow, the midpoint between everypoint on the front surface and every point on the back surface may notprecisely fall within a plane; as a practical matter, however, the TTV,warp and bow are typically so slight that to a close approximation themidpoints can be said to fall within an imaginary central plane which isapproximately equidistant between the front and back surfaces.

It is to be noted that the process of the present invention may besuccessfully carried out on silicon segments of varying thicknesses, thethickness of the material being in part a function of the type of deviceto be fabricated from it. For example, a relatively thin siliconsegment, such as a silicon wafer ranging in thickness from about 500 toabout 800 microns, may be used as the starting material. Alternatively,thicker segments, ranging in thicknesses from 800 microns up to about1500 microns or more, may be used. Typically, however, for theparticular devices of interest, such as thyristors and power diodes, aswell as low noise, high performance silicon detectors, thicknesses willrange from about 800 to about 1200 microns.

In a first embodiment of the process of the present invention, segment 1is heat-treated in an oxygen-containing atmosphere in step S₁ to grow asuperficial oxide layer 9 which envelopes segment 1. In general, theoxide layer will have a thickness which is greater than the native oxidelayer which forms upon silicon (about 15 Ångstroms); preferably, theoxide layer has a thickness of at least about 20 Ångstroms and, in someembodiments, at least about 25 Ångstroms or even at least about 30Ångstroms. Experimental evidence obtained to-date, however, suggeststhat oxide layers having a thickness greater than about 30 521 ngstroms,while not interfering with the desired effect, provide little or noadditional benefit.

In step S₂, the wafer is subjected to a heat-treatment step in which thewafers are heated to an elevated temperature to form and therebyincrease the number density of crystal lattice vacancies 13 in the bulk11 of wafer 1. Preferably, this heat-treatment step is carried out in arapid thermal annealer in which the wafers are rapidly heated to atarget temperature and annealed at that temperature for a relativelyshort period of time. In general, the wafer is subjected to atemperature in excess of 1150° C., preferably at least 1175° C., morepreferably at least about 1200° C., and most preferably between about1200° C. and 1275° C.

In the first embodiment of the present invention, the rapid thermalannealing step is carried out in the presence of a nitriding atmosphere,that is, an atmosphere containing nitrogen gas (N₂) or anitrogen-containing compound gas such as ammonia which is capable ofnitriding an exposed silicon surface. The atmosphere may thus consistentirely of nitrogen or nitrogen compound gas, or it may additionallycomprise a non-nitriding gas such as argon. An increase in the vacancyconcentration throughout the segment is achieved nearly, if notimmediately, upon achieving the annealing temperature. The segment willgenerally be maintained at this temperature for at least one second,typically for at least several seconds (e.g., at least 3), preferablyfor several tens of seconds (e.g., 20, 30, 40, or 50 seconds) and,depending upon the initial thickness and the desired resultingcharacteristics of the segment, for a period which may range up to about60 seconds (which is near the limit for commercially available rapidthermal annealers). The resulting segment will have a relatively uniformvacancy concentration (number density) profile.

Based upon experimental evidence obtained to-date, the atmosphere inwhich the rapid thermal annealing step is carried out preferably has nomore than a relatively small partial pressure of oxygen, water vapor andother oxidizing gases; that is, the atmosphere has a total absence ofoxidizing gases or a partial pressure of such gases which isinsufficient to inject sufficient quantities of siliconself-interstitial atoms which suppress the build-up of vacancyconcentrations. While the lower limit of oxidizing gas concentration hasnot been precisely determined, it has been demonstrated that for partialpressures of oxygen of 0.01 atmospheres (atm.), or 10,000 parts permillion atomic (ppma), no increase in vacancy concentration and noeffect is observed. Thus, it is preferred that the atmosphere have apartial pressure of oxygen and other oxidizing gases of less than 0.01atm. (10,000 ppma); more preferably the partial pressure of these gasesin the atmosphere is no more than about 0.005 atm. (5,000 ppma), morepreferably no more than about 0.002 atm. (2,000 ppma), and mostpreferably no more than about 0.001 atm. (1,000 ppma).

The rapid thermal anneal may be carried out in any of a number ofcommercially available rapid thermal annealing (“RTA”) furnaces in whichsilicon segments may be individually heated by banks of high powerlamps. RTA furnaces are generally capable of rapidly heating a siliconsegment, having a thickness within the ranges noted above, from roomtemperature to about 1200° C. in a few seconds. One such commerciallyavailable RTA furnace is the model 610 furnace available from AGAssociates (Mountain View, Calif.).

Intrinsic point defects (i.e., vacancies and silicon self-interstitials)are capable of diffusing through single crystal silicon, with the rateof diffusion being temperature dependant. The concentration profile ofintrinsic point defects at a given temperature, therefore, is a functionof the diffusivity of the intrinsic point defects and the recombinationrate. For example, intrinsic point defects are relatively mobile attemperatures in the vicinity of the temperature at which the wafer isannealed in the rapid thermal annealing step whereas they areessentially immobile for any commercially practical time period attemperatures of as much as 700° C. Experimental evidence obtainedto-date suggests that the effective diffusion rate of vacancies slowsconsiderably at temperatures less than about 700° C., and that perhapsat temperatures as great as 800° C., 900° C., or even 1,000° C., thevacancies can be considered to be immobile for any commerciallypractical time period.

Upon completion of step S₂, the wafer is rapidly cooled in step S₃through the range of temperatures at which crystal lattice vacancies arerelatively mobile in the single crystal silicon. As the temperature ofthe segment is decreased through this range of temperatures, thevacancies diffuse to the oxide layer 9 and become annihilated, thusleading to a change in the vacancy concentration profile with the extentof change depending upon the length of time the segment is maintained ata temperature within this range. If the segment were held at thistemperature within this range for an infinite period of time, thevacancy concentration would once again become substantially uniformthroughout the bulk 11, with the concentration being an equilibriumvalue which is substantially less than the concentration of crystallattice vacancies immediately upon completion of the heat treatmentstep. By rapidly cooling the segment, however, a non-uniformdistribution of crystal lattice vacancies can be achieved, with themaximum vacancy concentration being at or near central plane 7 and thevacancy concentration decreasing in the direction of the front surface 3and back surface 5 of the segment.

In general, the average cooling rate within this range of temperaturesis at least about 5° C. per second and preferably at least about 20° C.to about 30° C. per second, or more. Depending upon the desired depth ofthe low vacancy concentration region near the surface, the averagecooling rate may preferably be at least about 50° C. per second, stillmore preferably at least about 100° C. per second, with cooling rates inthe range of about 100° C. to about 200° C. per second being presentlypreferred for some applications. Once the segment is cooled to atemperature outside the range of temperatures at which crystal latticevacancies are relatively mobile in the single crystal silicon, thecooling rate does not appear to significantly influence the vacancyconcentration profile of the segment and, thus, does not appear to benarrowly critical. Conveniently, the cooling step may be carried out inthe same atmosphere in which the heating step is carried out.

In step S₄, platinum atoms are diffused into crystal lattice vacancies.In general, platinum is deposited on the surface of the silicon segmentand diffused in a horizontal surface by heating the segment for aspecified period of time. The diffusion time and temperature arepreferably selected such that the Frank-Turnbull mechanism dominatesplatinum diffusion. Furthermore, the diffusion time and temperature arepreferably sufficient to allow for the vacancy decoration by platinumatoms to reach steady-state.

For silicon segments having vacancy concentrations which are typical forthe present invention, the diffusion temperature typically ranges fromabout 650 to about 850° C. Preferably, however, the temperature rangesfrom about 670 to about 750° C. More preferably, the temperature rangesfrom about 680 to about 720° C. Diffusion time typically ranges fromabout 2 minutes to about 4 hours. Preferably, however, the time rangesfrom about 10 minutes to about 2 hours. More preferably, the time rangesfrom about 15 minutes to about 30 minutes.

Preferably, thermal diffusion of platinum is performed under anatmosphere comprising nitrogen or an inert gas, or mixtures thereof. Anoxygen-containing atmosphere may also be employed, given the lowtemperatures employed by the present process. However, it is to be notedthat generally this thermal diffusion step may be performed under anyatmosphere which, given the low temperatures employed, does not resultin the injection of point defects into the silicon segment matrix.

It is to be noted, however, that the precise time and temperature neededfor the diffusion process in order for platinum atoms to fully reactwith, or fill, vacancies present in the silicon segment may vary, atleast in part, as a function of the thickness of the sample and thenumber of vacancies which are present. As a result, the optimum time andtemperature may be determined empirically. For example, a siliconsegment may be divided into several portions and, after depositing thesame concentration of platinum onto each, heat treating each portionusing different time and temperature combinations.

Silicidation of the sample surface is preferably avoided becausesilicidation may result in the injection of point defects. Mechanicalstrain may also originate from the silicide layer, which may have anon-negligible influence on platinum diffusion. Additionally,silicidation of the sample surface may have an undesirable influence onplatinum detection or measurement techniques. Accordingly, to minimizethe potentially negative influence of the silicidation process, theplatinum deposition method preferably results in a surface concentrationof less than one monolayer, wherein a platinum monolayer corresponds toa surface area concentration of about 2×10¹⁵ atom/cm² Stated anotherway, it is preferred that a quantity of platinum be deposited onto thesilicon segment surface such that the resulting surface concentrationdoes not exceed about 2×10⁵ atoms/cm².

Platinum deposition may be achieved by essentially any method common inthe art, provided such methods do not result in the deposition of aquantity of platinum onto the surface of the silicon segment which mayresult in the creation of surface defects and the injection of pointdefects into the bulk of the segment. For example, sputtering or e-gunevaporation techniques may be used to deposit fractions of a monolayeronto the surface of the segment. Alternatively, an acidified platinumsolution, having a platinum concentration of about 1 gram/liter forexample, may be deposited onto the surface by spin coating or immersingthe segment into the solution, preferably after the surface of thesegment has been treated with a solution of HCl:H₂O₂:H₂O (1:1:6) atabout 80° C. for about 10 minutes.

After the in-diffusion heat treatment of step S₄ is complete, the depthprofile, or concentration profile, of the platinum in the siliconsegment may be determined using means common in the art, such as throughthe use of deep level transient spectroscopy (DLTS) measurements. In oneembodiment of the DLTS measurements, silicon segments samples are cutinto pieces of about 1 cm², beveled with angles of 1.17° and 2.86°, andpolished. A layer of silicon about 15 microns in thickness is thenetched from a surface using a etch solution comprising a 2:1:1 ratio ofHF (hydrofluoric acid, 50% solution) to HNO₃ (nitric acid, fuming) toCH₃CO₂H (acetic acid, glacial). Schottky contacts are deposited byevaporation of hafnium. For ohmic contacts at the backside, gallium isused.

The detection ranged of substitutional platinum concentration C_(s)depends on the dopant concentration D_(d), the relationship betweenthese may be expressed as C_(d)×10⁻⁴<C_(s)<C_(d)×10⁻¹.

Platinum diffusion techniques, as well as platinum detection methods,are further described elsewhere. See, for example, articles by Jacob etal., J. Appl. Phys., vol. 82, p. 182 (1997); Zimmermann and Ryssel, “TheModeling of Platinum Diffusion In Silicon Under Non-EquilibriumConditions,” J. Electrochemical Society, vol. 139, p. 256 (1992);Zimmermann, Goesele, Seilenthal and Eichiner, “Vacancy ConcentrationWafer Mapping In Silicon,” Journal of Crystal Growth, vol. 129, p. 582(1993); Zimmermann and Falster, “Investigation Of The Nucleation ofOxygen Precipitates in Czochralski Silicon At An Early Stage,” Appl.Phys. Lett., vol. 60, p. 3250 (1992); and, Zimmermann and Ryssel, Appl.Phys. A, vol. 55, p. 121 (1992).

As a result of the first embodiment of the present process, high vacancyregions located within the bulk of the silicon segment are formed andare subsequently decorated or filled by platinum atoms, which arediffused into the silicon segment in accordance with the F-T mechanism.As illustrated by FIG. 5, the resulting silicon segment is characterizedby a region 17 which contains a peak platinum concentration, theconcentration being substantially uniform. In addition, the siliconsegment contains regions 15 and 15′ which extend from the front surface3 and back surface 5, respectively, to a depth t and t′. As compared toregion 17 of the silicon segment, regions 15 and 15′ contain arelatively lower concentration of platinum (not shown) and, as a result,possess longer minority carrier recombination lifetimes than region 17.Platinum concentrations within these surface layers or regions may rangefrom a low concentration of less than about 1×10¹¹ atoms/cm², up toabout 1×10¹² atoms/cm², about 1×10¹³ atoms/cm², or even a peakconcentration of about 5×10¹³ (excluding the surfaces of the segment).By dividing the silicon segment into various zones or regions by meansof tailoring or controlling the vacancy concentration profile, atemplate is effectively created through which is written a pattern orprofile for the resulting platinum concentration, after in-diffusion iscomplete.

It is to be noted that, due to interactions that may occur on thesilicon segment surfaces between platinum and intrinsic point defectsthat may be present, references to regions of peak concentration and,comparatively, of lower concentrations are intended to exclude thesilicon surfaces. Stated another way, the silicon segment surfaces arenot to be considered when determining or evaluating the concentrationprofile of recombination centers, or when making any comparisons basedthereon.

In accordance with the process of the present invention, theconcentration of minority carrier recombination centers in region 17 isprimarily a function of the concentration of vacancies which arepresent, which in turn is a function of the heating step (S₂) andsecondarily a function of the cooling rate (S₃). Likewise, the depth t,t′ from the front and back surfaces, respectively, of the regions 15 and15′ is also a function of the vacancy concentration, which in turn isprimarily a function of the cooling rate through the temperature rangeat which crystal lattice vacancies are relatively mobile in silicon.Accordingly, given that the depth t, t′ increases with decreasingcooling rates, the cooling rate may be controlled such that depths of atleast about 10, 20, 30, 40, 50, 70 or even 100 microns are attainable.

In a second embodiment of the present invention, a neutral (i.e., anon-nitriding, non-oxidizing) atmosphere is used in the heating (rapidthermal annealing) and cooling steps, in place of the nitridingatmosphere of the first embodiment. Suitable neutral atmospheres includeargon, helium, neon, carbon dioxide, and other such non-oxidizing,non-nitriding elemental and compound gases, or mixtures of such gases.The neutral atmosphere, like the nitriding atmosphere, may contain arelatively small partial pressure of oxygen, i.e., a partial pressureless than 0.01 atm. (10,000 ppma), more preferably less than 0.005 atm.(5,000 ppma), more preferably less than 0.002 atm. (2,000 ppma), andmost preferably less than 0.001 atm. (1,000 ppma).

In a third embodiment of the present invention, step S₁ (the thermaloxidation step) is omitted and the starting silicon segment has no morethan a native oxide layer. Referring now to FIG. 6A, when such a segmentis annealed under a neutral atmosphere, as discussed in reference to thesecond embodiment above, generally results similar to those ofembodiments 1 and 2 are obtained. However, when such a segment isannealed in a nitriding atmosphere, such as that of the firstembodiment, the effect differs from that which is observed when asegment having an oxide layer which is greater in thickness than anative oxide layer (i.e., an “enhanced oxide layer”) is annealed innitrogen atmosphere.

When the segment containing an enhanced oxide layer is annealed in anitrogen atmosphere (FIG. 6A), a substantially uniform increase in thevacancy concentration is achieved throughout the segment nearly, if notimmediately, upon reaching the annealing temperature; furthermore, thevacancy concentration does not appear to significantly increase as afunction of annealing time at a given annealing temperature. Incontrast, as can be seen from FIG. 6B, if the segment has nothing morethan a native oxide layer on the surface and if the front and backsurfaces of the segment are annealed in nitrogen, the resulting segmentwill have a vacancy concentration (number density) profile which isgenerally “U-shaped” for a cross-section of the segment. As illustratedby FIG. 6B and FIG. 7 (curve A), this profile may subsequently bemodified as the segment cools down, taking on a generally “M-shaped”form, due to vacancy transport in the near surface region to the surfaceitself, which acts as a sink for these intrinsic point defects. Morespecifically, a maximum vacancy concentration will occur at or withinseveral microns or tens of microns from the front and back surfaces anda relatively constant and lesser concentration will occur throughout thebulk, with the minimum concentration in the bulk initially beingapproximately equal to the concentration which is obtained in siliconsegments having an enhanced oxide layer, after having been treated inaccordance with the present process (denoted as curve B in FIG. 7).Furthermore, an increase in annealing time will result in an increase invacancy concentration in silicon segments lacking anything more than anative oxide layer.

Accordingly, referring again to FIG. 5, when a segment having only anative oxide layer is annealed in accordance with the present processunder a nitriding atmosphere, the resulting peak concentration ofminority carrier recombination centers, after platinum diffusion, willinitially be located generally within regions 15 and 15′, while the bulk17 of the silicon segment will contain a comparatively lowerconcentration of recombination centers. Typically, these regions of peakconcentration will be located within several microns (i.e., about 5 or10 microns), or tens of microns (i.e., about 20 or 30 microns), up toabout 40 to about 60 microns, from the silicon segment surface.

Experimental evidence further suggests that this difference in behaviorfor silicon segments having no more than a native oxide layer and thosehaving an enhanced oxide layer can be avoided by including molecularoxygen or another oxidizing gas in the atmosphere. Stated another way,when silicon segments having no more than a native oxide layer areannealed in a nitrogen atmosphere, it is preferred that the atmosphereadditionally containing a small partial pressure of oxygen. Suchatmospheric conditions result in segments having only a native oxidelayer behaving the same as segments which have an enhanced oxide layer.Without being bound to any particular theory, it appears thatsuperficial oxide layers which are greater in thickness than a nativeoxide layer serve as a shield which inhibits nitridization of thesilicon. Nitridization is believed to result in the formation ofvacancies in, or the injection of vacancies into, the silicon matrix. Asa result, as may be seen by comparing FIGS. 6A and 6B, the peakconcentration of vacancies (and thus recombination centers also) isactually greater than the peak that might otherwise be observed, ifvacancies where not injected.

The oxide layer may therefore be present on the starting silicon segmentor formed in situ, by growing an enhanced oxide layer during theannealing step. If the latter approach is taken, the atmosphere duringthe rapid thermal annealing step preferably contains a partial pressureof at least about 0.0001 atm. (100 ppma), and more preferably a partialpressure of at least about 0.0002 atm. (200 ppma). Referring again toFIG. 6A, in this way results similar to those of the first and secondembodiments of the present invention may be obtained. However, for thereasons previously discussed, the partial pressure of oxygen preferablydoes not exceed 0.01 atm. (10,000 ppma), with partial pressures of lessthan 0.005 atm. (5,000 ppma), 0.002 atm. (2,000 ppma), and even 0.001atm. (1,000 ppma) being more preferred.

However, it is to be noted that as an alternative to utilizing anatmosphere having a partial pressure of oxygen, the silicon segment maysimply be annealed under an oxygen atmosphere after annealing under anitrogen atmosphere or a neutral atmosphere, as described in the aboveembodiments, is complete. The oxygen annealing step may be performedafter the segment has been allowed to cool or, alternatively, may beperformed at temperature (i.e., while the segment is still hot after theinitial thermal anneal step has been completed). Furthermore, thisoxygen anneal step may optionally be performed for any of theabove-described embodiments as a means by which to further tailor orprofile the vacancy concentration within the silicon segment and, assuch, the resulting platinum concentration.

Without being held to any particular theory, it is believed that oxygenannealing results in the oxidation of the silicon surface and, as aresult, acts to create an inward flux of silicon self-interstitials.This inward flux of self-interstitials has the effect of graduallyaltering the vacancy concentration profile by causing recombinations tooccur, beginning at the surface and then moving inward. A region of lowvacancy concentration may therefore be created having a depth which maybe optimized for the particular end use of the device which is to befabricated from the silicon segment. Referring now to FIGS. 6C and 6D,it may be seen that, depending upon the conditions which were utilizedprior to the oxygen anneal step, a number of different platinumconcentration profiles may ultimately be obtained (where curves Athrough E, respectively, schematically represent different profilesobtained under different oxygen anneal conditions of the presentinvention). More specifically, curves A through E of FIGS. 6C and 6Dillustrate the resulting platinum concentration profiles which may beobtained as a result of conducting the oxygen anneal step to alter thevacancy concentration profile, prior to platinum in-diffusion.

Referring to FIG. 6D, for a silicon segment having a peak concentrationof vacancies near the front and back surfaces, it is to be noted thatafter platinum in-diffusion a segment may be obtained in which thedistribution of recombination centers is non-uniform. More specifically,this segment contains a non-uniform distribution of recombinationcenters wherein the maximum concentration is in a region which isbetween the front surface and the central plane and nearer to the frontsurface than the central plane, with the concentration of the nucleationcenters increasing from the front surface to the region of maximumconcentration and decreasing from the region of maximum concentration tothe central plane.

For silicon segments having the peak concentration of vacancies withinthe bulk 17 of the silicon segment, the depth t and t′ of regions 15 and15′, respectively, may be selectively increased by controlling the rateat which oxidation of the surfaces occurs. The rate of oxidation is inturn dependent upon a number of factors, such as the atmosphericconditions, temperature and duration of this oxidation step. Forexample, the rate of oxidation will increase as the concentration ofoxygen in the atmosphere increases, with the rate being greatest whenpyrogenic steam is employed.

It is to be noted that the precise conditions for the oxidativetreatment may be empirically determined by adjusting the temperature,duration and atmospheric composition in order to optimize the depth tand/or t′. However, if something other than pure oxygen or pyrogenicsteam is employed in the present process, preferably the partialpressure of oxygen in the atmosphere will be at least about 0.0001 (100ppma), and more preferably at least about 0.0002 (200 ppma). In thisregard it is to be noted that the limitations placed upon the oxygencontent, or partial pressure, for the thermal anneal step S₂ are notapplicable for this optional step of the process. Furthermore, if thepeak concentration of vacancies, and thus minority carrier recombinationcenters, for region 17 (or regions 15 and 15′) is to be substantiallyretained, the temperature of this oxidative treatment is preferably inexcess of about 1150° C. More preferably, the temperature is at leastabout equal to the temperature employed during the thermal treatment ofstep S₂. Without being held to any particular theory, it is believe thatif the temperature is less than that employed during the thermaltreatment, the peak concentration of recombination centers in region 17may actually decrease because of the direct recombination of vacanciesand self-interstitials.

This separate oxidative treatment approach is an acceptable alternativeto controlling the vacancy concentration profile, and accordingly theplatinum concentration profile, by means of adjusting the rate ofcooling, as described in detail above. It may be preferred in somesituations because of the additional flexibility it provides. Inaddition, this approach is preferred when the depth of t or t′ is inexcess of tens of microns, or several tens of microns. Furthermore,subsequent oxidative treatment of the silicon segment, prepare by any ofthe above-described embodiments of the present invention, provides themeans by which to prepare a silicon segment containing a numberdifferent minority carrier recombination center concentration profiles,as schematically illustrated in FIGS. 6C and 6D.

In other embodiments of the present invention, the front and backsurfaces of the segment may be exposed to different atmospheres, each ofwhich may contain one or more nitriding or non-nitriding gases. Forexample, the back surface of the silicon segment may be exposed to anitriding atmosphere as the front surface is exposed to a non-nitridingatmosphere. In addition, one of the surfaces may be shielded duringtreatment. For example, referring now to FIGS. 8A through 8F, after thevacancy concentration profile has been established throughout thesilicon segment, one surface may be shielded while the other is thermalannealed in an oxygen atmosphere, as noted above. Shielding may beachieved, for example, by simultaneously annealing multiple segments(e.g., 2, 3 or more) while being stacked in face-to-face arrangement;when annealed in this manner, the faces which are in face-to-facecontact are mechanically shielded from the atmosphere during theannealing. Alternatively, a similar effect may be achieved (dependingupon the atmosphere employed during the rapid thermal annealing step andthe desired platinum concentration profile of the segment) by forming anoxide layer only upon the side of the segment at which platinumin-diffusion is desired. Shielding one of the surface of a siliconsegment, prepared in accordance with the present invention, in this waymay ultimately yield a segment which contains an asymmetricrecombination center concentration profile, as the examples in FIGS. 8Athrough 8F illustrate. More specifically, FIGS. 8A and 8F illustrateexamples of the resulting platinum concentration profiles which may beobtained as a result of conducting the oxygen anneal step to alter thevacancy concentration profile in this way, prior to platinumin-diffusion.

It is to be noted that, in order for the desired recombination centerconcentration profile to be present in the device ultimately fabricatedfrom the silicon segment, the present process will typically beincorporated into the device fabrication process at a point which isappropriately suitable for the objectives of the present invention to beachieved. Furthermore, the platinum in-diffusion step of the presentprocess is preferably performed immediately following the rapid thermalanneal and subsequent cool down steps, S₂ and S₃.

More specifically, it is preferred that the steps of the present processbe performed in sequential order, without any intervening treatments.However, if intervening treatments are necessary as part of the devicefabrication process, it is preferred that such treatments be for onlyshort periods of time and at low temperatures. More specifically, it ispreferred that any intervening treatment be performed at a temperatureless than about 1000° C. for a duration which is not sufficient toresult in an appreciable change in the vacancy concentration profile, orrecombination center profile, which has previously been established inthe silicon segment as a results of the present process.

The starting material for the process of the present invention may be apolished silicon segment, such as a silicon wafer, or alternatively, asilicon segment which has been lapped and etched but not polished. Inaddition, the segment may have vacancy or self-interstitial pointdefects as the predominant intrinsic point defect. For example, thesegment may be vacancy dominated from center to edge, self-interstitialdominated from center to edge, or it may contain a central core ofvacancy dominated material surrounded by an axially symmetric ring ofself-interstitial dominated material.

In view of the above, it will be seen that the several objects of theinvention are achieved.

As various changes could be made in the above compositions and processeswithout departing from the scope of the invention, it is intended thatall matter contained in the above description be interpreted asillustrative and not in a limiting sense.

1. A process for heat-treating a single crystal silicon segment toinfluence the concentration profile of minority carrier recombinationcenters in the segment, the silicon segment having a front surface, aback surface, a central plane between the front and back surfaces, asurface layer which comprises the region of the segment between thefront surface and a distance, D, measured from the front surface andtoward the central plane, and a bulk layer which comprises the region ofthe segment between the central plane and surface layer, the processcomprising: heat-treating the segment in an atmosphere to form crystallattice vacancies in the surface and bulk layers; controlling thecooling rate of the heat-treated segment to produce a segment having avacancy concentration profile in which the peak density is at or nearthe central plane with the concentration generally decreasing in thedirection of the front surface of the segment; and, thermally diffusingplatinum atoms into the silicon matrix of the cooled segment such that aplatinum concentration profile results which is substantially dependentupon the vacancy concentration profile.
 2. The process of claim 1wherein said heat-treatment to form crystal lattice vacancies comprisesheating the segment to a temperature in excess of about 1175° C. in anon-oxidizing atmosphere.
 3. The process of claim 1 wherein saidheat-treatment to form crystal lattice vacancies comprises heating thesegment to a temperature in excess of about 1200° C. in a non-oxidizingatmosphere.
 4. The process of claim 1 wherein said heat-treatment toform crystal lattice vacancies comprises heating the segment to atemperature in the range of about 1200° C. to about 1275° C. in anon-oxidizing atmosphere.
 5. The process of claim 1 wherein said coolingrate is at least about 20° C. per second through the temperature rangeat which crystal lattice vacancies are relatively mobile in silicon. 6.The process of claim 1 wherein said cooling rate is at least about 50°C. per second through the temperature range at which crystal latticevacancies are relatively mobile in silicon.
 7. The process of claim 1wherein said cooling rate is at least about 100° C. per second throughthe temperature range at which crystal lattice vacancies are relativelymobile in silicon.
 8. The process of claim 1 wherein platinum atoms arethermally diffused into the silicon matrix of the segment by heating thesegment to a temperature ranging from about 670 to about 750° C.
 9. Theprocess of claim 1 wherein platinum atoms are thermally diffused intothe silicon matrix of the segment by heating the segment from about 10minutes to about 2 hours.
 10. The process of claim 1 wherein prior toplatinum in-diffusion the heat-treated or cooled segment is subjected toa second heat-treatment in an atmosphere of pure oxygen or pyrogenicsteam, the temperature of said second heat-treatment being at leastabout equal to the temperature of the first.
 11. The process of claim 1wherein said heat-treatment to form crystal lattice vacancies comprisesthe steps of: (a) subjecting the segment to a first heat-treatment at atemperature of at least about 700° C. in an oxygen containing atmosphereto form a superficial silicon dioxide layer which is capable of servingas a sink for crystal lattice vacancies; and, (b) subjecting the productof step (a) to a second heat-treatment at a temperature of at leastabout 1150° C. in an atmosphere having an essential absence of oxygen toform crystal lattice vacancies in the bulk of the silicon segment.
 12. Aprocess for heat-treating a single crystal silicon segment to influencethe concentration profile of minority carrier recombination centers inthe segment, the silicon segment having a front surface and a backsurface, the front surface having only a native oxide layer presentthereon, and a central plane between the front and back surfaces, theprocess comprising: heat-treating the front surface of the segment in anitriding atmosphere to form crystal lattice vacancies in the segment;controlling the cooling rate of the heat-treated segment to produce avacancy concentration profile in the cooled segment in which a maximumconcentration is between the front surface and the central plane andnearer to the front surface than the central plane, the vacancyconcentration generally increasing from the front surface to the regionof maximum concentration and generally decreasing from the region ofmaximum concentration to the central plane; and, thermally diffusingplatinum atoms into the silicon matrix of the cooled segment such that aplatinum concentration profile results which is substantially dependentupon the vacancy concentration profile.
 13. The process of claim 12wherein said heat-treatment to form crystal lattice vacancies comprisesheating the segment to a temperature in excess of about 1175° C. in anon-oxidizing atmosphere.
 14. The process of claim 12 wherein saidheat-treatment to form crystal lattice vacancies comprises heating thesegment to a temperature in the range of about 1200° C. to about 1275°C. in a non-oxidizing atmosphere.
 15. The process of claim 12 whereinsaid cooling rate is at least about 20° C. per second through thetemperature range at which crystal lattice vacancies are relativelymobile in silicon.
 16. The process of claim 12 wherein said cooling rateis at least about 100° C. per second through the temperature range atwhich crystal lattice vacancies are relatively mobile in silicon. 17.The process of claim 12 wherein platinum atoms are thermally diffusedinto the silicon matrix of the segment by heating the segment to atemperature ranging from about 670 to about 750° C.
 18. The process ofclaim 12 wherein platinum atoms are thermally diffused into the siliconmatrix of the segment by heating the segment from about 10 minutes toabout 2 hours.
 19. The process of claim 12 wherein prior to platinumin-diffusion the heat-treated or cooled segment is subjected to a secondheat-treatment in an atmosphere of pure oxygen or pyrogenic steam, thetemperature of said second heat-treatment being at least about equal tothe temperature of the first.
 20. The process of claim 12 wherein saidheat-treatment to form crystal lattice vacancies comprises the steps of:(a) subjecting the segment to a first heat-treatment at a temperature ofat least about 700° C. in an oxygen containing atmosphere to form asuperficial silicon dioxide layer which is capable of serving as a sinkfor crystal lattice vacancies; and, (b) subjecting the product of step(a) to a second heat-treatment at a temperature of at least about 1150°C. in an atmosphere having an essential absence of oxygen to formcrystal lattice vacancies in the bulk of the silicon segment.